Joints in Concrete Slabs

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29 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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Joints in Concrete Slabs


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Q

Joints in Concrete Slabs

A

1. What are Joints?


Although concrete expands and contracts with changes in

moisture and temperature the general overall tendency is to

shrink and, therefore crack. Irregular cracks are unsightly

and difficult to maintain. Joints are simply preplanned cracks.


Some forms of joints are:


a. Control (contraction) joint
-

These j
oints are constructed to

create planes of weakness so that cracks will occur at the

desired location.


b. Isolation (expansion) joints
-

They separate or isolate slabs

from other parts of the structure such as walls, footings, or

columns, and driveway
s and patios from sidewalks, garage

slabs, stairs, light poles and other obstructions. They permit

movement of the slab and help minimize cracking caused

when such movements are restrained.


c. Construction joints
-

These are joints that are placed at the

end of a day’s work. In slabs they may be designed to permit

movement and/or to transfer load. Often in reinforced

concrete a conscious effort is made to clean the joint and

bond the next

day’s work.


2. Why Are Joints Constructed?


Concrete cracks cannot be prevented entirely, but they can

be controlled and minimized by properly designed joints,

because:


a. Concrete is weak in tension natural tendency to shrink

stresses develop and

cracks

and, therefore, has a if restrained, tensile are likely to occur.


b. At early ages, before the concrete dries out, most cracking

is caused by temperature changes or by the slight contraction

that takes place as the concrete sets and hardens. L
ater, as

the concrete dries it will shrink further and either additional

cracks may form or existing cracks may become wider.


c. Joints provide relief for the tensile stresses and are less

objectionable than random cracks.


3. How to Construct Joints


Joints must be carefully designed and properly constructed if

uncontrolled cracking of concrete flatwork is to be avoided.

The following recommended practices should be observed.


a. The maximum joint spacing should not exce
ed 30 times the

thickness. For example, in a 100mm thick slab, the joints

should be no further apart than 3 meters.


b. All panels should be square or nearly so. The length should

not exceed 1.5 times the width. L
-
shaped panels should be

avoided.


c.

The joint groove should have a minimum depth of 1/4 the

thickness of the slab, but not less that 25mm. Tooled joints

must be run early in the finishing process and rerun later to

assure groove bond has not occurred.


d. Control joints can be tooled du
ring finishing or sawed with

a carborundum blade at an early age. Sawing should

commence as soon as excessive ravelling is observed.


e. Premolded joint filler, building paper or polyethylene

should

be used to isolate slabs from building walls or footings.

At least 50mm of sand over the top of a footing will also

prevent bond to the footing.


f. To isolate columns from slabs, form circular or square

openings which will not be filled until after t
he floor has

hardened. Slab control joints should intersect at the openings

for columns. If square openings are used around columns

the; square should be turned at 45 degrees to have the

control joints intersect at the diagonals of the square.


g. If
the slab contains wire mesh, cut out alternate wires

across control joints. Note that wire mesh will not prevent

cracking. Mesh tends to keep the cracks and joints tightly

closed.


h. Construction joints key the two edges of the slab together,

either
to provide transfer of loads or to help prevent curling

or warping of the two adjacent edges. Galvanized metal keys

are preferred for interior slabs, however, a beveled 25mm by

50mm strip, nailed to bulkheads or form boards, can be used

in slabs that are at least 125 mm thick to form a key which

will resist vertical loads and movements. Metal dowels can

also be used in slabs that will carry heavy loads. Dowels must

be carefully lined up and parallel or they may induce restraint

and ca
use random cracking at the end of the dowel.


i. Joints in industrial floors subject to heavy traffic require

special attention to avoid spalling of joint edges. Such joints

should be filled with a material capable of supporting joint

edges. Manufactur
er’s recommendations and performance

records should be checked before use.


Follow These Rules for Proper Jointing


1. Plan exact location of all joints before construction.


2. Provide isolation joints between slabs and columns, walls

and footing, an
d at junctions of driveways with walks, curbs

or other obstructions.


3. Provide control joints and joint filling materials as outlined

in specifications.



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